As an example of an energy source that makes available energy in amounts that vary over the service life of the energy source, consider an energy scavenger combined with an energy storage that stores the scavenged energy. The expressions “energy scavenging” and “energy harvesting” refer to technologies, known in the art, for converting ambient, stray energy from external sources into electricity. For example, a piezoelectric crystal can be used to convert mechanical energy from vibrations into electric energy. As another example, a temperature difference between two different electrical conductors can be used to generate a voltage across, or current through, the junction of the conductors. As still another example, consider a magnet that is made to move past a coil as a result of the vibratory or cyclic motion of a mass to which the magnet is attached. The moving magnet generates a change of magnetic flux in the coil, and induces an electromotive force on the coil. Accordingly, a plurality of technologies is available for extracting energy from the environment. A component, which extracts stray energy from its environment and makes the extracted energy available for consumption, is referred to as an energy scavenger.
For completeness, the terms “vary” and “varying” as used herein in order to qualify the energy source discussed above, cover the scenarios wherein the amount of energy made available varies over a range, and also cover the scenario of a complete lack of energy.
The extracted energy may be stored in an energy storage device, such as a (rechargeable) battery, a capacitor or a supercapacitor. A supercapacitor is also known as an electric double-layer capacitor. A supercapacitor is an electrochemical capacitor that has a much higher energy density than a conventional capacitor, owing to the use of a layer of nano-porous material that dramatically increases the layer's surface area, allowing many more charge carriers to be stored in any given volume.
A typical application of using an energy scavenger is the powering of small autonomous electronic devices, e.g., sensors, or actuators. These devices are small and require little power. Their applications are limited by the reliance on electric power. Scavenging energy from ambient vibrations, wind, heat or light, enables to replenish the energy consumed by the electronic device and enables the device to continue functioning over a time span that is long enough compared to the intended application. Such devices can be exploited in, e.g., condition monitoring applications.
US patent application publication 2008/0047363 discloses a device that can be attached to a structure or live subject and that can harvest energy from its environment to power sensing, storing and transmitting data about the structure or live subject. The known device comprises an energy harvester and an energy storage device for storing the energy harvested. The known devices also comprises sensors, conditioners for conditioning the sensor signals, a processor for processing data representative of the conditioned sensor signals, data storage and a data transmitter. The known device comprises a microcontroller that controls the power consumed by the sensors, signal conditioning, processing, and transmission components of the energy harvesting wireless sensing device. The power consumed by all of the known device's components (sensor, conditioner, processor, data storage, and data transmission) must be compatible with the amount of energy harvested. Minimizing the power required to collect and transmit data correspondingly reduces the demand on the power source. Reduced power consumption is inherently beneficial to the performance of systems using harvested energy. A reduction in power consumption can be realized through the use of embedded software in the microcontroller that controls the power consumed by the sensors, signal conditioning, processing, and transmission components of the energy harvesting wireless sensing device. By adjusting the time these components are on, for example, power consumed can be reduced. In addition the microcontroller can be programmed to process and store sensed information, rather than immediately transmit the sensed information, so as to reduce the frequency of data transmission events. The microcontroller is programmed to go into sleep mode and turn power to other components off. The microcontroller drains a small amount of energy in the sleep mode to keep a timer running. When it is time to wake up, the microcontroller assesses the level of the energy available at the energy storage device. If the level is too low, the microcontroller goes back into sleep mode. If the level is okay, the microcontroller restores power to the other components, performs a predetermined task and goes back to sleep again.